This invention relates to an improved method of determining small concentrations of chemical compounds by means of plasma chromatography and to an apparatus suitable for practicing the method of this invention.
Because of the ever growing concern with environmental problems, it is frequently necessary to monitor low concentrations of pollutants in ambient air or in industrial process gases such as, for example, smoke stack effluents and reactor vents. Government regulations set maximum permissible concentrations of many pollutants, sometimes in terms of parts per billion (ppb).
Plasma Chromatography is particularly well suited for the determination of minute amounts of various chemical species, even in the ppb range. Most fundamental work in the field of plasma chromatography has been done by Franklin GNO Corporation, West Palm Beach, Florida. U.S. Pat. Nos. 3,812,355 to Wernlund et al., 3,845,301 to Wernlund et al., and 3,621,239 to Cohen are representative of the prior art. In a plasma chromatograph, a gas stream carrying one or more chemical substances (gases or vapors) is exposed to an ionization source such as, for example, a radioactive material. Ionizable molecules in the gas stream form ions, which are allowed to drift through the so-called "drift tube" of a plasma chromatograph between a charged shutter grid and a collector at the other end of the drift tube. Various ions present in the drift tube at any time separate according to the ion mobilities, which in turn depend, among others, on the ion mass, size, and shape. The amplitude of the ion current for any given drift time does not necessarily vary in direct proportion to the molecular concentration because there is competition for charge among all species present in the sample gas; so that the ion current amplitude for a given ionic species X may vary, even if the concentration of X is constant, because it is affected by the concentrations of other ionized species in the gas stream, which may not be constant. In order to improve the accuracy of measuring by plasma chromatography low concentrations of a chemical substance in a gas sample with reasonable accuracy, it is necessary to account for variations in concentrations of background species.